Searching for a color singleton among new items

A difficult visual search can be facilitated if some distractors appear in advance (preview benefit). The preview benefit is thought to result from top-down inhibition of old distractor locations, limiting search to new elements. An alternative account assumes bottom-up capture of attention by the sudden onset of new elements. The purpose of the present study was to shed new light on the mechanism underlying the preview benefit. In a series of experiments, the time course of target and distractor processing in preview search was investigated using event-related brain potentials (ERPs). Results show that although target selection as reflected by the N2pc component is more efficient when half of the distractors are presented prior to the remaining elements, the presence of old distractors interferes with search. The allocation of attention to old distractor locations was probed during preview and after the new elements appeared. Early ERP components in response to the probe flashed during the preview interval did not differ from two neutral preview conditions, suggesting that old distractors were passively viewed. After the onset of new elements, new distractors initially received more attention than old distractors, but this difference decreased over time. These findings cast considerable doubt on the view that old distractors are inhibited during the preview interval. Instead, the pattern of results is consistent with an onset capture account of the preview benefit.